This invention relates generally to fan case containment systems and methods of fabrication, and more specifically to fiber reinforced composite fan case containment systems.
Gas turbine engines, and particularly turbofan engines used in aircraft, have a fan with a hub and a plurality of fan blades disposed for rotation about a central axis. Catastrophic damage can occur to the aircraft and its occupants if a broken fan blade is propelled radially outwardly from the rotating hub under centrifugal force and impacts on the aircraft fuselage.
To prevent such damage, it is common to include a generally cylindrical fan case about the periphery for containing the fan blade. For smaller diameter engines, adequate containment capability may be achieved with a metallic, hard-wall, case thick enough to resist penetration by blade fragments. However, for larger diameter engines, a metallic case thick enough to resist penetration is prohibitively heavy. Therefore, it is known in the art to utilize what are termed “soft-wall” containment systems.
In a soft-wall system, a lightweight, high strength ballistic fabric may be wrapped in multiple layers around a relatively thin support structure. In operation, a separated blade or blade fragment locally penetrates the support structure and strikes the fabric. The fabric layers capture and contain the fragment. Conventional support structures may be fabricated of aluminum based on weight considerations. The support structure may include aluminum honeycomb structures.
U.S. Pat. No. 6,053,696 proposes a composite shell for a gas turbine engine fan case. The composite shell disclosed therein includes two end support hoops structurally bonded to a center portion having an open lattice structure. Other examples of fiber reinforced composite structures and fabrication methods are provided in the following U.S. patents: U.S. Pat. No. 4,086,378 to Kam et al shows a cylindrical composite structure with helical, axial and circumferential reinforcing ribs forming an interior lattice; U.S. Pat. No. 4,012,549 to Slysh describes a high strength composite structure with an isogrid lattice of equilateral triangles; and U.S. Pat. No. 4,137,354 to Mayes, Jr. et al describes a ribbed composite cylindrical structure and manufacturing process.
In composite softwall fan containment case design, structural integrity during and after a fan blade out (FBO) event is a critical design requirement. Aluminum honeycomb used in a more conventional softwall containment case design might not provide sufficient residual stiffness and strength. Also, bonding net shape aluminum honeycomb to composite inner shell is a costly manufacturing process.
Softwall fan containment case design is usually lighter weight than a hardwall containment case design. However, hardwall case is usually less costly to fabricate since the complete casing structure can be fabricated in one single process without the additional cost associated with containment fabric.
Accordingly, it would be desirable to have a containment system incorporating advantages of a lighter weight softwall containment case while providing simplified fabrication processes.